1. Technical Field
The present invention relates to a prismatic battery, such as a lithium-ion or alkaline battery. More particularly, the invention relates to a prismatic battery including an electrode group contained in a prismatic metal outer can. The electrode group has a positive electrode substrate exposed part at one end and a negative electrode substrate exposed part at the other end.
2. Related Art
Lithium secondary batteries with high energy density (measured in Wh/Kg) have been developed as power sources for cellular phones, notebook computers, compact camcorders, and other mobile electronics and communications equipment, and for electric vehicles (EVs) and hybrid electric vehicles (HEVs). In particular, prismatic batteries have been attracting attention for their high volume energy density (measured in Wh/l).
A typical method for manufacturing such prismatic batteries can be illustrated as follows. JP-A-2004-303500 is an example of related art. A positive electrode substrate typically made of aluminum foil is coated with a positive electrode mixture containing a positive electrode active material to make a strip positive electrode plate. A negative electrode substrate typically made of copper foil is coated with a negative electrode mixture containing a negative electrode active material to make a strip negative electrode plate. The strip positive and negative electrode plates are placed so as to face each other with a strip separator interposed therebetween to form a multilayer structure. This structure is rolled to make a flattened electrode group and contained in a prismatic metal outer can typically made of aluminum. Then a nonaqueous electrolyte is injected into the can, which completes a prismatic battery.
The thus-manufactured flattened electrode group 20, illustrated in FIG. 5, has a positive electrode substrate exposed part 21a (not coated with the positive electrode mixture) extended from the positive electrode plate at one end and a negative electrode substrate exposed part 21b (not coated with the negative electrode mixture) extended from the negative electrode plate at the other end. Welded to the side of the positive electrode substrate exposed part 21a is a positive electrode current collector 22 provided with a welded positive electrode lead 22a. Welded to the side of the negative electrode substrate exposed part 21b is a negative electrode current collector 23 provided with a welded negative electrode lead 23a. 
An insulator 24 is placed on the side of the positive electrode current collector 22, while an insulator 25 is placed on the side of the negative electrode current collector 23. Subsequently, the flattened electrode group 20 to which the positive and negative electrode leads 22a and 23a are welded is contained in a prismatic metal outer can 27. The positive electrode lead 22a is welded to the lower end of a positive electrode terminal 26a on a sealing plate 26, while the negative electrode lead 23a is welded to the lower end of a negative electrode terminal 26b. The sealing plate 26 is welded to an opening edge of the metal outer can 27. Then, a predetermined electrolyte is injected from an injection hole 26c on the sealing plate 26 and thereafter the hole 26c is plugged with an injection plug, which completes a prismatic battery. The sealing plate 26 also has a gas vent valve 26d. 
With this prismatic battery disclosed in the related art, the positive and negative electrode substrate exposed parts 21a and 21b are exposed at both ends of the flattened electrode group 20. This structure requires a clearance between the sides of the flattened electrode group 20 in its thickness direction and the inner wall of the metal outer can 27 in its thickness direction in order to prevent an internal short circuit between the exposed parts 21a and 21b via the outer can 27. However, providing a clearance that is wide enough to prevent an internal short circuit means extra space in the battery, resulting in an undesirable decrease in the battery's volume energy density.
In addition, since the insulator 24 is simply placed on the side of the positive electrode current collector 22 and the insulator 25 is simply placed on the side of the negative electrode current collector 23, the positive and negative electrode substrate exposed parts 21a and 21b may come in contact with the metal outer can 27, thereby causing an internal short circuit, if the battery falls and the flattened electrode group 20 moves, for example. This short circuit can be prevented by providing an insulating resin tape, for example, to the periphery of the flattened electrode group 20 including the outer surfaces of the insulators 24 and 25 and to the inner wall of the outer can 27.
This however causes another problem. Since the insulating resin tape has low mechanical strength, it may be broken or otherwise damaged when the flattened electrode group 20 provided with this tape is inserted into the metal outer can 27, and as a result, comes in contact with the opening surface of the metal outer can 27. Furthermore, providing this tape at more positions requires more work. In this case, the sides of the positive electrode current collector 22 including the positive electrode substrate exposed part 21a and of the negative electrode current collector 23 including the negative electrode substrate exposed part 21b may be covered by a resin molded piece. This molded piece, however, would be large in size, which increases the cost of parts and makes it difficult to have a reduced thickness, thereby lowering the battery's volume energy density.